In electrophotography, an electrostatic latent image is generally formed on a photoreceptor comprising a photocondutive substance, etc. and developed with a developer to form a visible image. The resulting visible image is then fixed by heat or a solvent.
A mixture of a fine powder comprising a resin and a colorant, called a toner, and fine iron powder or ferrite powder, called a carrier, is used as the developer for electrophotography.
Since the photoconductive layer can be charged positively or negatively, a positive or negative electrostatic latent image can be obtained when the original is exposed to light. Hence, when a negatively charged electrostatic latent image is developed with a positively charged developer, a positive-positive image corresponding to the original is formed. Generally, the developer is a fine powder obtained by mixing a colorant such as a dye or a pigment with a synthetic resin. The charging characteristics of the developer are influenced by the synthetic resin which is the principal ingredient.
Usually, a charge controlling agent is added to the developer to obtain desired friction charging characteristics.
Generally, particles comprising a colorant such as carbon or a color pigment dispersed in a thermoplastic resin or further containing a charge controlling agent for controlling the quantity of charge are used as toners for electrophotography. Many charge controlling agents such as nigrosine, aniline black, quaternary ammonium salts and metal complex salt dyes are conventionally used. Usually, these charge controlling agents are dispersed in the resin by adding carbon and the charge controlling agent to the resin molten by heating and kneading the resulting mixture. In fine particles such as particles of carbon or the charge controlling agent, however, primary particles are generally apt to aggromerate, thereby forming secondary particles. Further, they are affected by compatibility with the resin and have a difficulty in uniformly mixing with the resin. Thus, various problems with regard to the characteristics of the toners are caused.
Particularly, it is very important whether the charge controlling agent is uniformly dispersed in the resin. It is conventionally considered that various characteristics of the toners are greatly affected by the dispersibility of the charge controlling agent. To solve these problems, a twin screw heating kneader which applies intensively shear has to be used, or kneading with heating has to be carried out for many hours at present. However, these methods have disadvantages in that much electric heating rates are required, productivity is low, and the cost of equipment is high.
Further, toners prepared by these methods are still insufficient with regard to the uniform dispersion of the charge controlling agent and carbon in the resin, and hence problems with regard to unstable charging performance and scattering still remain unsolved.
JP-A-4-281467 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a method wherein carbon and the charge controlling agent are mechanically dry-mixed with each other, and the resulting composition is used to prapare a toner, thereby improving the durability and charge stability of the toner.
However, the method disclosed therein has problems in that carbon and the charge controlling agent can not be uniformly mixed, an effect of improving various performances of the toner is insufficient, the quantity of charge is lowered when used for many hours, and the scattering of the toner and a defect in the image occur.
Accordingly, an object of the present invention is to provide a charge controlling agent composition wherein the charge controlling agent and carbon are uniformly dispersed, thereby solving the problems associated with the performances of conventional toners.
Another object of the present invention is to provide a toner comprising the resin, the charge controlling agent and carbon uniformly dispersed therein, which has good charging characteristic stability and does not suffer from scattering and a defect in the image.